Combining Measurements from Stationary and Mobile Platforms Over the Los Angeles Basin to Comprehensively Evaluate the Weather Research and Forecasting Model During SUNVEx

The role of topographic circulations in redistributing pollutants in complex-terrain within an urban environment is a difficult but important problem in air-quality. In 2021, a multi-institutional effort addressed the impact of urban emissions in the complex terrain surrounding Los Angeles (LA), California and Las Vegas, Nevada from June through August in the Southwest Urban NOx and VOCs Experiment (SUNVEx). The effort combined unique measurement strategies by using a truck-based system to measure in-situ chemistry, and horizontal and vertical winds from a micro-pulsed Doppler lidar system. Primary goals of the mission were to characterize and quantify source emissions in urban environments and to characterize the role of the topographically induced winds in the LA and La Vegas regions on the redistribution of pollutants both locally and non-locally. Here we focus on data collected over the LA basin during drives targeting the sea-breeze along a north-south track, basin flows across LA in the east-west direction, and a loop to address situations where flows over low ridges can lead to transport above the boundary layer (BL).

Data collected over LA during August are used to evaluate the performance of different configurations of the Weather Research and Forecasting (WRF) model to determine a version best suited for predicting days of poor air quality. In addition to data from the mobile system, supporting measurements from commercial aircraft and surface stations over land and water are used to interpret the spatiotemporal characteristics of local wind regimes that develop in the afternoon, potentially leading to transport out of the LA basin. Results are examined statistically to determine biases in different model configurations near the surface as well as throughout the boundary layer into the free troposphere. Data acquired along three different driving configurations gives an opportunity to look at regional biases and potentially link model discrepancies to distance from the coast, elevated terrain, land-use, and surface roughness associated with a varied urban layout. A case study compares lidar-measured and WRF-modeled representations of the horizontal and vertical structure of a seabreeze event across the LA basin with respect to time, to determine structural representativeness of the model results.